The invention relates to a radar apparatus provided with a transmitter for the periodic transmission of mutually disjunct groups of N radar transmitter pulses, with N=4, 5, 6, . . . , a receiver for the receipt of echo signals of the radar transmitter pulses and a video processor for detecting possible objects on the basis of the received echoes and for estimating parameters of these objects.
Radar apparatus of this type are well-known in the art. The devices in question are radar apparatus in which a listening time is observed between two successive radar transmitter pulses to enable the receipt of echo signals. The drawback of these known radar apparatus is that, for distant targets, the interval between two pulses shall be chosen to be large which, however, would preclude the unambiguous determination of a target""s radial velocity.
The inventive radar apparatus meets this drawback to a significant extent and is characterized, according to an aspect of the invention, in that the video processor is designed to process echoes in a listening time observed between two groups of radar transmitter pulses. For one target, N echoes are received on the basis of which both target range and radial velocity can be determined unambiguously.
A favourable embodiment of the radar apparatus according to the invention is characterized in that the video processor is provided with memory means for storing a row of complex video strengths determined by the receiver. This row can be searched for a pattern of echo signals that corresponds to the transmitted pattern. Besides, by considering the phase shift of the N echo signals, it is possible to determine the Doppler frequency and, thus, the radial velocity of the target associated with the echo signals.
A further favourable embodiment of the radar apparatus according to the invention is characterized in that the video processor is provided with a digital filter for processing a continuous subrow from the row of video strengths. This entails the advantage that the subrow may be considerably shorter than the row, which allows the use of a digital filter with a limited number of inputs. More precisely: the subrow shall be so long to enable the simultaneous processing of the N echo signals reflected by a point target. It will then suffice to pass the row of complex video strengths through the digital filter in order to obtain a substantially optimal detection.
A very favourable embodiment according to a further aspect of the invention is characterized in that the digital filter comprises a Fourier transformer or an equivalent linear transformer. Not only does this enable a substantially optimal detection but also renders it possible to simultaneously determine, on the basis of the phase shift of the N echo signals, the Doppler frequency and, thus, the radial velocity of a target.
A further favourable and simple embodiment of the radar apparatus according to the invention is characterized in that within a group, the pulses are mutually phase-coherent and a time interval between two pulses is always a multiple of a unit of time.
A favourable embodiment according to a further aspect of the invention is characterized in that the: transmitter is designed to position the pulses in a group in order of time in such a manner that for a point target the Fourier transformer delivers an output signal for at least substantially one subrow.
A favourable embodiment according to a further aspect of the invention is characterized in that out of all possible suitable pulse patterns, a pulse pattern is selected for which in case of a point target, the Fourier transformer delivers an output signal with minimum side lobes.
The invention furthermore relates to a method for operating a radar apparatus according to which groups of N radar transmitter pulses are periodically transmitted, with N=4, 5, 6, . . .
According to an aspect of the invention, the inventive method is characterized in that, in a listening time between two groups of radar transmitter pulses, the received radar echo signals are applied to a video processor to estimate parameters of a possible detected target.
A favourable realization of the inventive method is characterized in that the received radar echo signals are digitized and sequentially passed through a digital filter with at least N inputs for combining the N echo signals reflected by a target.
According to a further aspect of the invention, a favourable realization of the method is characterized in that within a group, the radar transmitter, pulses are transmitted non-equidistantly in time, in which case the radar transmitter pulses within a group are positioned such that at least for a point target, the digital filter delivers output signal at least substantially once.
A favourable realization of the method according to a further aspect of the invention is characterized in that the received radar echo signals are digitized and sequentially applied to a Fourier transformer with N inputs and M outputs, where M greater than  greater than N. The velocity of a target can then be determined at the same time. The radar transmitter pulses within a group are preferably positioned such that at least for a point target, the Fourier transformer delivers an output signal with minimum side lobes.
A favourable realization according to a further aspect of the invention is characterized that for each transmitted group and each subrow a predetection is generated for at least the strongest output signal of the Fourier transformer or an equivalent transformer. A detection is generated if at least P practically identical predetections are generated for the same subrow from Q consecutively transmitted groups, with P=1, 2, . . . , Q=1, 2, and Pxe2x89xa6Q.